The present invention relates to underground mining in general and more specifically to a system and method for developing gateroad entries in a longwall mining system.
Underground mining of a coal body is usually accomplished by cutting or driving a plurality of entries into a coal seam to divide it into various discrete panels. The sizes and configurations of the various discrete panels are made in accordance with a mine development plan that takes into account many factors, including, for example, the overall shape and configuration of the coal seam, ventilation, roof control, haulage, and escapeway requirements, as well as considerations relating to maximum resource recovery. Additional considerations may take into account various mine services such as power, water, and drainage. Once the sizes and configurations of the various panels have been determined, a plurality of gateroad entries are then driven into the coal seam to define the panels. The coal in the panels may then be removed by any of a number of various mining systems, such as continuous or longwall. The particular mining system that is used to extract the coal from the panels also depends on a variety of factors, such as, for example, the characteristics of the coal seam and surrounding strata, as well as on surface effects, such as the amount of allowable surface subsidence and, of course, overall costs.
In the continuous mining system, a continuous mining machine or "continuous miner" is used to mechanically break the coal and load it for transport onto a shuttle car or a continuous haulage system. The maximum distance that the face can be advanced on a single cut with the continuous miner is limited by roof control and ventilation requirements, as well as on equipment capability. In most continuous mining systems, the foregoing considerations limit the maximum amount that the face can be advanced in a single cutting sequence to about 30 to 40 feet. Consequently, the name "continuous mining system" is somewhat of a misnomer, since the face can only be advanced by relatively short distances before having to stop to extend roof support and ventilation to the newly advanced face.
The longwall mining system differs substantially from the continuous mining system described above. While the continuous mining system is used to cut relatively small tunnels or entries into a coal seam, the longwall mining system is used to remove an extremely large block of coal, or panel, in a single, continuous operation. While the size of the panel may vary depending on the particular coal seam, most longwall panels tend to be rather large, having widths in the range of 500 to 1,000 feet and lengths of 6,000-15,000 feet or more. However, before the longwall panel can be mined, it must first be outlined within the coal seam by gateroad entry sets.
Referring now to FIG. 1, a longwall panel 14 is shown as it could be outlined from the coal seam by gateroad entry sets developed by a continuous mining system. In a typical development process, the longwall panel 14 is outlined by driving a pair of gateroad entry sets 12 into the coal seam 78. While a gateroad entry set 12 may comprise two, three, or four entries arranged in generally parallel, spaced-apart relation, a typical gateroad entry set 12 may comprise three entries, such as entries 17, 18, and 19. The roof of each gateroad entry set 12 is supported by a plurality of pillars 20 that are defined between adjacent pairs of entries 17, 18, 19, and cross-cuts or breaks 22. The parallel gateroad entry sets 12 are connected at one end by a set of mains or submains 16 and at the other end or tailgate by a set of bleeders 24. The longwall panel 14 is thus defined between the pair of gateroad entry sets 12, the mains or submains 16, and the bleeders 24. After the panel 14 is defined, the longwall mining equipment (not shown) may then be moved into position adjacent the face 26 of panel 14. A suitable continuous haulage system (also not shown) may be installed in the entry immediately adjacent the panel 14 and used to haul away the coal removed by the longwall machine. As the longwall machine traverses across the face 26 of longwall panel 14, the face 26 is advanced in the direction of arrow 28 until the entire panel 14 is removed. The type of longwall mining system just described is referred to as "retreat" longwall mining, since the panel face 26 moves in a direction back toward the mains or submains 16.
As mentioned above, the entries 17, 18, 19 and cross cuts 22 of each gateroad entry set 12 are formed by a continuous mining system. Since the maximum advance of the continuous mining system in a single cutting sequence is necessarily limited to 30 or 40 feet, the development of the gateroad entry sets 12 is a slow process, requiring several sequential cuts. Referring now to FIG. 2, a first entry 17 may be driven into the coal seam by a continuous miner 25 to a depth equal to the maximum allowable advance, typically 30-40 feet. The continuous miner 25 then must be removed and roof bolts installed and ventilation advanced to the new face. A second cut may then be initiated by maneuvering the continuous miner 25 into a position adjacent the first cut. After advancing the second cut by the maximum advance length, the continuous miner 25 is again moved and roof support and ventilation moved to the new face defined by the second cut. The process is then repeated until one side of a pillar 20 has been defined. A second entry 18 in parallel, spaced-apart relation to the first entry 17 may then be advanced in a similar manner, removing the continuous miner and providing roof support and ventilation between face advances. After driving the second entry 18 a sufficient distance to define the opposite side of the pillar 20, a cross-cut or break 22 may then be cut between the first and second entries 17, 18 to define the pillar 20. Of course, the roof of the cross cut 20 must also be supported, usually by roof bolts. Next, a third entry 19 may be advanced into the field in parallel, spaced-apart relation to the second entry 18. A cross-cut or break 22 is then cut between the second and third entries 18 and 19 to define another pillar 20. Each gateroad entry set 12 is thus developed by cutting the various entries 17, 18, 19 and cross cuts 22 in an intermittent, grid-like fashion until the gateroad entry set 12 extends into the coal seam 78 for the full length of the longwall panel 14.
As the foregoing description makes clear, the development of the entry sets 12 required to define a longwall panel 14 having a length of 6,000-15,000 feet is by no means trivial, and represents a significant amount of the total cost, time, and manpower required to mine the longwall panel 14. The intermittent and grid-like fashion in which the gateroad entry sets 12 are mined significantly increases the ratio of total drivage per foot of section advance, and it is not unusual for a continuous miner to drive 4 to 5 feet to advance the entry set 12 a single foot in a three entry system of the type shown in FIG. 1. If the length of the longwall panel 14 is to be 15,000 feet, the continuous miner used to develop the entry sets must drive 60,000 to 75,000 feet for each gateroad entry set 12.
While such slow and inefficient entry set development has heretofore been tolerated, improvements in the longwall mining process have increased longwall mining rates to the point where gateroad entry set development cannot keep pace with longwall mining rates. Indeed, the ability to drive the number of entries required to develop new longwall panels is now the rate-determining step.
Another disadvantage associated with the currently practiced gateroad entry set development process is that the pillars 20 used to support the roof of each gateroad entry set 12 cannot be later removed. The inability to remove the coal pillars decreases the overall coal extraction ratio of the operation, i.e., the ratio of coal removed to the amount of coal remaining in-situ. Obviously, such decreased extraction ratios represent decreased utilization of the mineral resource and, usually, increased cost.
Consequently, there remains a need for a gateroad entry development system that can keep pace with the ever increasing longwall mining rates, but without increasing the cost, time, or total manpower required to develop the gateroad entries. However, the increased development rates should not be made at the expense of decreasing the extraction ratio. Additional advantages could be realized if such a gateroad entry development system decreased the costs associated with ventilation and roof support.